1. Field of the Invention
The present invention generally relates to gallium-nitride based light emitting diodes and, more particularly, to the structure and fabrication method of a transparent metallic conductive layer for enhancing the gallium-nitride based light emitting diodes' luminous uniformity.
2. The Prior Arts
A gallium-nitride (GaN) based light-emitting diode (LED) having a conventional structure as shown in FIG. 1 is usually grown on a sapphire substrate. The conventional GaN-based LED contains a number of GaN-based epitaxial layers on top of a side of the sapphire substrate 101. These epitaxial layers are sequentially stacked from bottom to top in the following order on the sapphire substrate 101: a low-temperature GaN buffer layer 102, a high-temperature GaN buffer layer 103, an n-type GaN ohmic contact layer 104, an indium-gallium-nitride (InGaN) active layer 105, a p-type aluminum-gallium-nitride (AlGaN) cladding layer 106, a p-type GaN ohmic contact layer 107, and a p-type transparent metallic conductive layer 108. Then, on top of the p-type transparent metallic conductive layer 108, there is a p-type metallic electrode 109. Additionally, on top of the n-type GaN ohmic contact layer 104, there is an n-type metallic electrode 110.
Within this conventional GaN-based LED, the GaN-based epitaxial layers (i.e., the LED itself), the sapphire substrate 101, and the resin packaging material (not shown) have refraction indices 2.4, 1.77, and 1.5 respectively. Due to such a variation in terms of the refraction indices, only up to 25% of the lights generated by the InGaN active layer 105 could directly escape from the GaN-based LED. The rest 75% of the lights are confined by a waveguide structure formed by the sapphire substrate 101 and the resin packaging material. The 75% of the lights also have a high probability to be absorbed again after undergoing multiple reflections within the LED and, therefore, cannot be effectively utilized. In other words, the light emitting efficiency of the conventional GaN-based LED is inherently limited by the re-absorption of the transparent metallic conductive layer and the LED's internal epitaxial structure.
In addition, as the p-type GaN ohmic contact layer 107 has a rather low conductivity with its resistivity coefficient generally between 1-2 Ωcm for a thickness between 0. 1-0.5 μm, the electrical current is confined in area having a lateral distance about 1 μm under the p-type metallic electrode 109. Therefore, to distribute the electrical current evenly so as to achieve uniform lighting, the p-type transparent metallic conductive layer 108 is formed on top of the p-type GaN ohmic contact layer 107 and covers the entire light emitting area. To enhance its transparency, the p-type transparent metallic conductive layer 108 has to be rather thin. The p-type transparent metallic conductive layer 108 therefore is usually made of Ni and Au, and has a thickness between 50 and 500 Å.
According to researches on the transparent metallic conductive layer made of Ni and Au, in order to lower the GaN-based LED's working voltage, the contact resistivity between the p-type transparent metallic conductive layer and the p-type ohmic contact layer has to be reduced effectively. On the other hand, to enhance the GaN-based LED's external quantum efficiency, the p-type transparent metallic conductive layer must have at least 80% transparency for visible lights whose wavelength is between 400-700 nm. According to an article published in Applied Physics Letters (Vol. 74, 1999, p 1275), a NiO semiconducting intermediate layer formed by annealing in an oxygenic environment could effectively reduce contact resistivity and enhance transparency. Another article published on Solid-state Electronics (Vol. 47, 2003, p 1741) pointed out that, to effectively enhance transparency, the thickness of Ni and Au should be as small as possible while, to effectively reduce contact resistivity, the thickness of Ni and Au should be as large as possible.
To effectively disrupt the foregoing waveguide structure so as to enhance the LED's luminous efficiency, the p-type ohmic contact layer 107's surface could be arranged to have a texturing surface. FIG. 2 is a schematic diagram showing an enlargement of the interface 120 between the p-type transparent metallic conductive layer 108 and the p-type ohmic contact layer 107 as depicted in FIG. 1. As shown in FIG. 2, Ni and Au are used to form the p-type transparent metallic conductive layer 108 on the texturing surface. When alloyed in an oxygenic environment, the Au layer 108a is very easy to have an uneven distribution within the NiO layer 108b. This would cause uneven lateral distribution of electrical current, localized light emission, and increase of working voltage.
Therefore, using Ni and Au bilayered structure to form the p-type transparent metallic conductive layer on the texturing surface of GaN-based LED's p-type ohmic contact layer, according to the foregoing limitations, still has room for improvement.
Accordingly, there is a need for a better structure for forming the p-type transparent metallic conductive layer so that not only the luminous uniformity of the texturing surface could be improved, the GaN-based LED's working voltage and external quantum efficiency could be enhanced as well.